1. Field of Invention
This invention relates to a device for controlling sound volume and tone of input audio signals and, more particularly, to a device for reducing the noise generated when sound volume or tone are regulated.
2. Related Art
A known sound volume control device disclosed in Japanese Patent Laid-Open No. 7-142944 (JP-A-07142944) comprises a sound volume regulating circuit for regulating sound volume according to a set of control data including the electronic volume and is adapted to reduce the switching noise generated at the time of regulating the sound volume by updating the control data to be fed to the volume regulating circuit in response to the output of a zero-crossing detection circuit arranged in the device for detecting zero-crossings in the input audio signal.
There have also been known similar devices comprising, in addition to a volume regulating circuit, a tone regulating circuit for regulating the tone according to a set of control data, using a zero-crossing detection circuit for detecting zero-crossings in the input audio signal.
FIG. 2 of the accompanying drawings shows a block diagram of a known volume/tone regulating circuit provided with a zero-crossing detection circuit. Referring to FIG. 2, the circuit comprises an input terminal 1 for receiving audio signals that are AC signals oscillating to alternately move into positive and negative zones relative to a reference voltage; a volume regulating circuit 2 for regulating the volume of the audio signal S received via the input terminal 1 according to a set of volume control data BD; a tone regulating circuit 3 for regulating the tone of the audio signal BS that has been regulated for volume according to a set of tone control data TD and transmitting the regulated audio signal TS to an output terminal 4; an interface circuit 6 for receiving the data transmitted from a microcomputer, transferring the data serially to a downstream shift register 7, and generating a storage completion signal E after the transfer; first and second latch circuits 8 and 9 for latching the volume control data BD and the tone control data TD out of the data stored in the shift register 7 according to write signals L1 and L2; and first and second decoders for decoding the output data of the first and second latch circuits 8 and 9 and feeding the decoded outputs to the volume regulating circuit 2 and the tone regulating circuit 3, respectively.
The volume/tone regulating circuit further comprises a zero-crossing detection circuit 12 for receiving the audio signal S applied to the input terminal 1 and detecting zero-crossings in the input audio signal by comparing the signal with a reference voltage, a storage completion flip-flop (FF) 13 to be set for operation by a storage completion signal E, and a signal generating circuit 14 for producing write signals L1 and L2 respectively to the first and second latch circuit 8 and 9 in response to the detection output of the zero-crossing detection circuit 12 after the generation of an output by the storage completion FF 13.
When volume control data BD and tone control data TD are transmitted from the microcomputer, the interface circuit 6 stores the transmitted data in the shift register 7 and generates a storage completion signal E to set the storage completion FF 13 for operation. Once the zero-crossing detection circuit 12 detects zero-crossings in the input audio signal S and generates a detection signal after the storage completion FF 13 is set for operation, the write signal generating circuit 14 outputs write signals L1 and L2 so that the new volume control data BD and the new tone control data TD stored in the shift register 7 are latched respectively to the first and second latch circuit 8 and 9 in order to update the existing volume control data BD and the tone control data TD.
Since the volume regulating circuit of the known device is designed to simply modify the extent of attenuation, no phase shift takes place before or after the regulating operation and, therefore, the zero-crossings of the input audio signal agrees with those of the output audio signal. Thus, the volume regulating circuit is satisfactorily effective for noise reduction.
On the other hand, however, since the tone regulating circuit modifies gain according to the frequency, the audio signal shows an advanced or retarded phase after the regulating operation. Therefore, the zero-crossings of the input audio signal do not agree with those of the output audio signal produced after regulation. In other words, If the tone is modified at zero-crossings of the input audio signal, a switching noise can be generated due to an abrupt change in the signal because the tone is modified at points that do not agree with zero-crossings of the output audio signal.